The Myrtle formation in southwestern Oregon is herein considered to be a group, as originally proposed by Louderback, and is subdivided into the Riddle and Days Creek formations. Some of the Myrtle formation as mapped by Diller is excluded from the Myrtle group and is reassigned to the Dothan formation. The Riddle formation consists dominantly of gray siltstone, is about 1,000 ft. thick, and contains fossils of latest Jurassic (Portlandian - middle to late Tithonian) age. The Days Creek formation consists mostly of alternating units of greenish gray massive sandstone and siltstone, is at least 2,000 ft. thick, and contains fossils of Early Cretaceous (Valanginian to Barremian) age. The Riddle formation rests with angular unconformity on the older Jurassic Galice, Rogue, and Dothan formations. The Days Creek formation rests concordantly on the Riddle formation and locally overlaps onto older Jurassic rocks. The contact between the Riddle and Days Creek formations is considered to be a disconformity involving the Berriasian and part of the Valanginian stages.

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